The study provides preliminary assessments of the formation and development of debris flows in Vietnam (. Tran et al., 2004) following a number of field surveys under the national project while referring to previous studies of colluvium in Asia and elsewhere in the world (. Bauziene, 2000; Irfan and Tang, 1992; Lai, 2011; Gray, 2008; Fleming and Arvid, 1994). | Vietnam Journal of Earth Sciences 38(3), 257-266 Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences (VAST) Preliminary assessments of debris flow hazard in relation to geological environment changes in mountainous regions, North Vietnam Tran Van Tu*, Dao Minh Duc, Nguyen Manh Tung, Van Duy Cong Institute of Geologial Sciences, Vietnam Academy of Sciences and Technology Received 7 April 2016. Accepted 15 August 2016 ABSTRACT Debris flow, widely viewed by geo-scientists as a special combination of landslide and flash flood, causes devastating damages to people and environment in northern mountainous regions of Vietnam. Field observations in the areas damaged by debris flows in northern Vietnam identified types of soils and rocks that were more likely to cause debris flows. Unlike flash floods, almost debris flows occurred at the end of the rainy season when soils and rocks were water-oversaturated thus mechanically weak; this is when pore water pressure decreases, lowering the strength from the soil. Landslides causing debris flows are commonly current slides. The tip of a landslide is often confined within a stream that has a permanent or seasonal flow. Debris flows mainly occur in proluvium, colluvial deposits or tectonic breccia zones. However, not a debris flow initiated in a tectonic breccia zone has been recorded in the northern mountainous regions of Vietnam. Colluvial deposits have been intensively investigated by many researchers worldwide. These deposits are commonly formed in neo-tectonic active zones, weak bed rocks, particularly old metamorphic rocks such as sericite shale, terrigenous and Cenozoic or late Mesozoic volcanic sedimentary rocks that are distributed at steep slopes and/or highly differentiated reliefs. These features appear to be a prerequisite for the exogenous processes, including rolling stones, falling rocks, landslides and surface erosions to occur. To study the mechanical .
